U.S. patent application number 15/185938 was filed with the patent office on 2016-11-03 for implantable medical device.
This patent application is currently assigned to Medtronic, Inc.. The applicant listed for this patent is Medtronic, Inc.. Invention is credited to Mark J. Conroy, John Kast, Erik R. Scott, Carl D. Wahlstrand.
Application Number | 20160317806 15/185938 |
Document ID | / |
Family ID | 38372484 |
Filed Date | 2016-11-03 |
United States Patent
Application |
20160317806 |
Kind Code |
A1 |
Kast; John ; et al. |
November 3, 2016 |
IMPLANTABLE MEDICAL DEVICE
Abstract
A method of manufacturing an implantable medical device having
reduced MRI image distortion, includes producing an implantable
medical device. The implantable medical device has a configuration
that comprises a housing and one or more internal components
disposed within the housing. The configuration is based upon a
design process that includes creating a first prototype,
determining the aggregate relative magnetic permeability of the
first prototype, and modifying the design of the first prototype by
at least one of (a) selecting and adding a diamagnetic shimming
material to the first prototype or (b) repositioning one or more
internal components of the first prototype. Modifying the design
results in a modified design that is the configuration for the
implantable medical device.
Inventors: |
Kast; John; (Hugo, MN)
; Wahlstrand; Carl D.; (St. Paul, MN) ; Conroy;
Mark J.; (St. Louis Park, MN) ; Scott; Erik R.;
(Maple Grove, MN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Medtronic, Inc. |
Minneapolis |
MN |
US |
|
|
Assignee: |
Medtronic, Inc.
Minneapolis
MN
|
Family ID: |
38372484 |
Appl. No.: |
15/185938 |
Filed: |
June 17, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14582817 |
Dec 24, 2014 |
9393408 |
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15185938 |
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14040136 |
Sep 27, 2013 |
8923969 |
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14582817 |
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13398588 |
Feb 16, 2012 |
8548591 |
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14040136 |
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11728030 |
Mar 23, 2007 |
8131368 |
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13398588 |
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60785881 |
Mar 24, 2006 |
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60827621 |
Sep 29, 2006 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61N 1/3718 20130101;
H01M 10/052 20130101; Y02E 60/10 20130101; A61N 1/3605 20130101;
Y10T 29/49 20150115; Y10T 29/49002 20150115; A61N 1/0551 20130101;
H01M 10/0525 20130101; H01M 2/0275 20130101; H01M 2/22 20130101;
H01M 2/0285 20130101; H01M 10/0587 20130101; A61N 1/3758 20130101;
H01M 2220/30 20130101; A61N 1/086 20170801; G01R 33/3875 20130101;
G01R 33/56572 20130101; H01M 2/263 20130101; A61B 34/10 20160201;
H01M 2/026 20130101; H01M 2/0217 20130101; A61N 1/378 20130101;
A61B 5/055 20130101; H01M 2/30 20130101; A61N 1/36125 20130101;
A61N 1/37514 20170801; H01M 2/1066 20130101; H01M 10/0431 20130101;
A61N 1/05 20130101; A61N 1/0529 20130101; H01M 2/0404 20130101 |
International
Class: |
A61N 1/08 20060101
A61N001/08; A61N 1/375 20060101 A61N001/375; A61N 1/378 20060101
A61N001/378; A61B 5/055 20060101 A61B005/055; G01R 33/3875 20060101
G01R033/3875; H01M 2/10 20060101 H01M002/10; H01M 2/02 20060101
H01M002/02; H01M 10/0525 20060101 H01M010/0525; H01M 10/0587
20060101 H01M010/0587; H01M 2/26 20060101 H01M002/26; H01M 2/30
20060101 H01M002/30; A61N 1/36 20060101 A61N001/36; G01R 33/565
20060101 G01R033/565 |
Claims
1. A method of manufacturing an implantable medical device having
reduced MRI image distortion comprising: producing an implantable
medical device having a configuration that comprises a housing and
a plurality internal components disposed within the housing;
wherein the configuration is based upon a design process that
includes: for at least two internal components, identifying their
magnetic permeabilities; and positioning the at least two internal
components proximate each other if they have opposite magnetic
permeabilities, or spreading apart the at least two internal
components from each other within the housing if they are both
diamagnetic or both paramagnetic.
2. The method of claim 1, wherein the design process further
includes: creating a first prototype; determining the aggregate
relative magnetic permeability of the first prototype; and
modifying the design of the first prototype by repositioning one or
more internal components of the first prototype, wherein said
modifying results in a modified design that is the configuration
for the implantable medical device.
3. The method of claim 2, wherein the design process further
includes: creating a first electromagnetic model to determine the
aggregate relative magnetic permeability of the first prototype;
and creating a second electromagnetic model of the modified design
to determine the aggregate relative magnetic permeability of the
modified design.
4. The method of claim 2, wherein the step of modifying the design
includes repositioning the one or more internal components
according to their magnetic permeabilities.
5. A method of obtaining an MRI image with reduced distortion
caused by an implanted medical device, the method comprising:
positioning a patient having an implanted medical device in an MRI
machine for image acquisition; prior to imaging, using shim coils
to address magnetic field inhomogeneity caused by at least one of
the patient or the implantable medical device; acquiring an image
with the MRI machine; and reconstructing the image using a custom
image processing routine that adjusts for known image distortion
associated with the implanted medical device of the patent.
6. The method of claim 5, wherein the processing routine executes
an image processing algorithm that compensates for known factors of
the implanted medical device that cause the known image distortion,
the known factors being at least one of material composition, size,
and location of the implanted medical device.
7. The method of claim 5, wherein the processing routine executes
an image processing algorithm that compensates for image distortion
caused by the implanted medical device, the processing algorithm
having been determined experimentally according to previously
acquired images of the implanted medical device or equivalent.
8. The method of claim 5, wherein the step of using shim coils
includes at least one of manually adjusting the shim coils or
automatically adjusting the shim coils using software.
9. The method of claim 5, wherein the step of acquiring an image is
performed utilizing a spin echo pulse sequence.
10. The method of claim 5, wherein the step of using shim coils is
performed to address magnetic field inhomogeneity caused by the
implantable medical device.
11. The method of claim 10, wherein the processing routine executes
an image processing algorithm that compensates for image distortion
caused by the implanted medical device, the processing algorithm
having been determined according to at least one of previously
acquired images of the implanted medical device or equivalent, or
known factors of the implanted medical device that cause the known
image distortion.
12. The method of claim 11, wherein the step of using shim coils
includes at least one of manually adjusting the shim coils or
automatically adjusting the shim coils using software.
13. The method of claim 12, wherein the step of acquiring an image
is performed utilizing a spin pulse echo sequence.
Description
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS
[0001] The present application is a divisional of U.S. patent
application Ser. No. 14/582,817, filed Dec. 24, 2014, which is a
divisional of U.S. patent application Ser. No. 14/040,136 (now U.S.
Pat. No. 8,923,969), filed Sep. 27, 2013, which is a Continuation
of U.S. patent application Ser. No. 13/398,588 (now U.S. Pat. No.
8,548,591), filed Feb. 16, 2012, which is a Continuation of U.S.
patent application Ser. No. 11/728,030 (now U.S. Pat. No.
8,131,368), filed Mar. 23, 2007, which claims the benefit under 35
U.S.C. .sctn.119(e) of U.S. Provisional Patent Application No.
60/785,881 filed on Mar. 24, 2006 and U.S. Provisional Patent
Application No. 60/827,621 filed Sep. 29, 2006. The entire
disclosures of each of the foregoing applications are incorporated
herein by reference.
BACKGROUND
[0002] The present inventions relate generally to the field of
implantable medical devices (IMDs). More particularly, the present
inventions relate to IMDs such as implantable neurological
stimulation (INS) devices that include features intended to reduce
magnetic resonance imaging (MRI) distortion.
[0003] Implantable neurological stimulation devices (sometimes
referred to as an implantable neuro stimulator or INS) generate
electrical stimulation signals that are used to influence the human
nervous system or organs. Conventionally, the INS has been
surgically implanted into a patient in a subcutaneous pocket in the
abdomen, pectoral region, or upper buttocks area. Electrical
contacts carried on the distal end of a lead are placed at the
desired stimulation site (e.g., at a location in the spine or
directly in the brain) and the proximal end of the lead is
connected to the INS.
[0004] It may be desirable to implant the INS at a location in the
patient's head in cases where the distal end of the lead is
provided at a site directly in the brain. For example, it may be
desirable to implant the INS under the scalp of the patient's head
(either on top of the surface of the skull or in a pocket or cutout
formed in the skull).
[0005] One difficulty with implanting medical devices such as INS
devices within the body of a patient is that the materials used in
such devices may tend to alter or distort images produced during
MRI scans. Such distortion may extend beyond the immediate
surrounding area of the device.
[0006] Accordingly, there is a need to provide an implantable
medical device such as an INS that exhibits reduced image
distortion when MRI scans are taken as compared to conventional
devices. There is also a need to provide an improved implantable
medical device that utilizes different materials for components to
minimize MRI image distortion. There is further a need to provide
an improved method of performing an MRI scan that provides less
image distortion as compared to conventional scanning methods.
SUMMARY
[0007] An exemplary embodiment relates to an implantable medical
device includes a housing formed of a first material and a first
electronic component provided within the housing. The implantable
medical device also includes a second material provided in contact
with at least a portion of the housing. At least one of the housing
and the first electronic component has a magnetic permeability in a
magnetic field that differs from the magnetic permeability of
water. The second material is provided in an amount effective to
reduce MRI image distortion caused by the implantable medical
device.
[0008] Another exemplary embodiment relates to an implantable
medical device that includes a housing and a plurality of
electronic components provided within the housing. The housing and
electronic components comprise materials that characteristically
produce image distortion in magnetic resonance imaging scans. A
material is provided in contact with at least a portion of the
housing for reducing the image distortion.
[0009] Another exemplary embodiment relates to an implantable
medical device that includes a housing having a shimming material
provided in contact with an inner surface of the housing to reduce
the amount of image distortion caused by the implantable medical
device when the device is subjected to MRI scans.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIGS. 1A through 1F illustrate MRI image distortion caused
by the use of a titanium material.
[0011] FIGS. 2A through 2F illustrate MRI image distortion caused
by the use of a ferrite material.
[0012] FIGS. 3A through 3F illustrate MRI image distortion caused
by the use of a steel material.
[0013] FIG. 4 is a cutaway perspective view of an implantable
medical device in the form of an INS according to an exemplary
embodiment.
[0014] FIG. 5 is a partial cutaway perspective view of a battery
configured for use in an INS shown such as that shown in FIG. 4
according to an exemplary embodiment.
[0015] FIG. 6 is an exploded perspective view of the battery shown
in FIG. 5 according to an exemplary embodiment.
[0016] FIG. 7 is a schematic plan view of a hybrid or circuit board
in which components that may cause MRI image distortion are not
provided adjacent one another.
[0017] FIG. 8 is a schematic cross-sectional view of an implantable
medical device according to an exemplary embodiment in which a
potting material or adhesive includes a shimming material provided
therein.
[0018] FIG. 9 is a schematic cross-sectional view of an implantable
medical device according to an exemplary embodiment illustrating a
shimming material provided as a plating provided on the inner
surface of the device housing.
[0019] FIG. 10 is a schematic cross-sectional view of an
implantable medical device according to an exemplary embodiment
illustrating a shimming material provided as a plating provided on
the outer surface of the device housing.
[0020] FIG. 11 is a schematic cross-sectional view of an
implantable medical device according to an exemplary embodiment
illustrating a shimming material provided on a portion of the
device housing or casing.
[0021] FIG. 12 is a schematic cross-sectional view of an
implantable medical device according to an exemplary embodiment
illustrating a shimming materials having differing thicknesses
provided on the device housing.
[0022] FIG. 13 is a schematic sectional plan view of an implantable
medical device according to an exemplary embodiment.
[0023] FIG. 14 is a schematic cross-sectional view of the device
shown in FIG. 13 illustrating a shimming material coated or plated
on the device housing in locations other than those in which
recharge or telemetry coils are provided.
[0024] FIG. 15 is a drawing illustrating several possible coating
or plating patterns that may be used to provide a shimming material
on an implantable medical device.
[0025] FIG. 16 is a schematic cross-sectional view of an
implantable medical device according to an exemplary embodiment
having shimming components coupled to the device housing.
[0026] FIG. 17 is a schematic plan view of an implantable medical
device having shimming components provided on the exterior surface
of the device according to an exemplary embodiment.
[0027] FIG. 18 is a schematic cross-sectional view of an
implantable medical device according to another exemplary
embodiment.
[0028] FIG. 19 is a schematic plan view of an implantable medical
device according to an exemplary embodiment having shimming
components provided in a header block.
[0029] FIG. 20 is an MRI image of a battery housing or casing made
of stainless steel.
[0030] FIG. 21 is an MRI image of a battery housing or casing made
of an aluminum housing.
DETAILED DESCRIPTION
[0031] According to an exemplary embodiment, an implantable medical
device (e.g., an INS) is provided that includes features intended
to reduce the amount of MRI image distortion. According to one
exemplary embodiment, one or more components of the device may
utilize materials or coatings having predefined magnetic
permeabilities that are intended to reduce or offset image
distortion. According to another exemplary embodiment, one or more
components of the device may be configured to reduce the occurrence
of eddy currents generated during an MRI scan (e.g., by reducing or
varying the thickness of various components).
[0032] The human body is composed mostly of water, which has a
relative permeability of 0.9999912. Accordingly, water is slightly
diamagnetic (see, e.g., Table 1).
TABLE-US-00001 TABLE 1 Classification of Materials Classification
Diamagnetic Paramagnetic Ferromagnetic Permeability <1 >1
>>1
[0033] When the human body undergoes an MRI scan, materials having
magnetic permeabilities that differ greatly from that of water when
the magnetic field is applied will cause distortion in the
resulting MRI image. It has been observed that the larger the
difference between the magnetic permeability of the material and
the magnetic permeability of water at the relatively high magnetic
field strengths generated during an MRI scan, the greater the image
distortion will be.
[0034] For example, FIGS. 1A and 1B illustrate an image taken
during an MRI scan (at a magnetic field strength of 1.5 Tesla) of a
20 cm by 40 cm phantom cylinder filled with water to simulate a
human head without an implanted INS. FIGS. 1C and 1D are images
taken using a spin echo MRI scan of the phantom cylinder with a
titanium can taped to one side of the cylinder. Areas 5 of image
distortion are visible in the images as relatively dark portions.
Such distortion is increased when using a gradient echo MRI scan,
as shown in FIGS. 1E and 1F.
TABLE-US-00002 TABLE 2 Relative magnetic permeabilities of selected
materials Material Relative Permeability Water 0.9999912 Copper
0.9999906 Silver 0.9999736 Lead 0.9999831 Air 1.00000037 Oxygen
1.000002 Aluminum 1.000021 Titanium 6-4 1.00005 (Grade 5) Palladium
1.0008 Platinum 1.0003 Manganese 1.001 Cobalt 250 Nickel 600 Iron
280,000
[0035] Table 2 lists relative magnetic permeabilities for various
materials at relatively low magnetic field strengths (data obtained
from various publicly available sources such as
http://www.matweb.com). As seen in Table 2, while titanium has a
relative permeability of 1.00005, iron (a major component of steel)
has a relative permeability of 280,000. As such, it would be
expected that including iron in an INS would result in relatively
significant image distortion. In general, materials that are
ferromagnetic at room temperature--iron, cobalt, and nickel--have
relative permeabilities that greatly exceed that of water at
relatively high magnetic field strengths, and thus would be
expected to distort MRI images far more than materials that have
magnetic permeabilities closer to that of water.
[0036] FIGS. 2 and 3 provide further illustrations of this
principle. In contrast to the relatively minor distortion shown in
FIGS. 1C through 1F where a titanium can was coupled to the phantom
cylinder, FIGS. 2 and 3 illustrate MRI image distortion resulting
from the use of a ferrite material (FIG. 2) and a steel material
(FIG. 3) in place of the titanium can (FIGS. 2A, 2B, 3A, and 3B are
control images similar to those shown in FIGS. 1A and 1B). As shown
in FIGS. 2 and 3, for ferrite and steel materials, the resultant
image distortion is far greater than was present using the titanium
material.
[0037] While the data in Table 2 illustrates relative magnetic
permeabilities of various materials, the inventors have observed
that certain materials may exhibit different characteristics at the
relatively high field strengths used in MRI scans. For example,
while Table 2 suggests that copper is slightly diamagnetic at low
field strengths, it has been observed that the permeability of
copper tends to shift toward being relatively paramagnetic at
higher field strengths. In selecting materials as described below
(e.g., for use in shimming implantable medical devices), the
high-field-strength permeabilities of the materials should be
assessed to confirm that such materials will provide the desired
effect when subjected to the relatively high magnetic field
strengths used in MRI scans.
[0038] According to an exemplary embodiment, an implantable medical
device may be designed such that it produces less MRI image
distortion than would be exhibited by a conventional device.
Various different design criteria may be employed in order to
accomplish this goal. For example, according to one exemplary
embodiment, the amount of ferromagnetic materials (e.g., nickel,
cobalt, iron, and ferrites) used in the device may be reduced.
While all metals would be expected to produce some degree of MRI
image distortion, reducing the amount of ferromagnetic materials
would have a greater effect on image distortion than reducing the
amount of slightly diamagnetic and paramagnetic materials.
[0039] It may also be desirable to minimize the eddy currents that
may be induced during imaging. Eddy currents cause distortion,
although such distortion is not expected to be as significant as
that caused by ferromagnetic materials. Reducing the eddy currents
associated with a particular device may be accomplished principally
by reducing or varying the thickness of various components as
compared to conventional components.
[0040] It may also be desirable to remove or shim non-ferromagnetic
materials that have permeabilities that differ from that of water.
For example, relatively large components that are made of
paramagnetic materials may be shimmed with a diamagnetic material
to balance out the paramagnetic material and to reduce image
distortion. According to an exemplary embodiment, materials that
have permeabilities that differ from water at high field strengths
(e.g., 1.5 or 3 Tesla) such as those found in MRI imaging
applications may be shimmed, removed from the device, or made of
materials less likely to cause significant image distortion.
[0041] FIG. 4 illustrates an implantable medical device 100 in the
form of an implantable neurological stimulation (INS) device
according to an exemplary embodiment. As shown in FIG. 4, the
device 100 includes a battery 10, coils 120 (e.g., for recharging
the battery or for telemetry), a wiring board or hybrid 130 (e.g.,
a circuit board having various electronic components soldered or
otherwise provided thereon), a device enclosure 140 (e.g., a
housing or casing), interconnects 150, and a connector or header
160 (the connector includes an aperture as shown at the right side
of FIG. 4 that is configured to have a lead inserted therein).
[0042] FIGS. 5 and 6 provide further illustrations of the
components of the battery 10 that may be used in conjunction with
device 100. It should be noted that the battery 10 may be a
non-rechargeable (i.e., primary) or a rechargeable (i.e.,
secondary) lithium-based battery according to various exemplary
embodiments.
[0043] With reference to FIG. 5, the battery 10 includes a battery
case or housing 20 provided in the form of a relatively thin-walled
hollow structure that is configured for having a plurality of
components provided therein. A liner 24 is provided adjacent or
proximate the housing 20 to separate internal components of the
battery 10 from the housing 20. A cover or cap 22 is provided at a
top surface of the battery 10 and may be coupled (e.g., welded,
adhered, etc.) to the housing 20. A headspace insulator 26 is
provided within the housing 20 to provide a space in which
connections may be made to electrodes provided within the housing
20 (additionally, a coil liner 27 as shown in FIG. 6 may be
provided which may act to separate a cell element from the
headspace region of the battery 10). A member or element 29 in the
form of a bracket may be provided to couple a current collector of
a negative electrode to the case and/or to the housing.
[0044] The battery 10 includes a cell element 30 provided within
the housing 20 that comprises at least one positive electrode and
at least one negative electrode. Each of the electrodes has a
member or element in the form of a tab or current collector coupled
thereto. As illustrated in FIGS. 5 and 6, electrode 32 is a
positive electrode having tab 34 coupled thereto, and electrode 36
is a negative electrode having a tab 38 coupled thereto. In such a
configuration, the battery 10 has a case negative design. According
to other exemplary embodiments in which the battery includes a case
positive or neutral design, the electrodes may be reversed (e.g.,
electrode 32 would be the negative electrode and electrode 36 would
be the positive electrode).
[0045] The current collector 34 of the positive electrode is
electrically coupled to a pin or terminal 25 (e.g., a feedthrough
pin) that is provided such that it protrudes through an opening or
aperture 23 provided in the cover 22. In such an embodiment, the
pin would act as the positive terminal for the battery and may be
connected at a distal end to a feature included in a hybrid (either
directly or indirectly). In a case neutral or case positive battery
design, the pin 25 may be coupled to a tab provided on a negative
electrode.
[0046] A separator 40 is provided intermediate or between the
electrodes, and an electrolyte 50 is also provided in the housing
20 (e.g., through an opening or aperture 28 in the form of a fill
port provided in the cover 22 of the battery 10).
[0047] In conventional lithium batteries used in implantable
medical device applications, the housing of the battery is made
from a metal such as stainless steel. To reduce the image
distortion caused by the battery 10, the housing 20 may be made of
a material that is less likely to result in substantial MRI image
distortion such as one or more of the following materials: (1)
aluminum or an aluminum alloy (including the 3000 series); (2)
aluminum foil/polymer composites with heat-sealable edges (i.e, a
"foil pack"); (3) titanium and titanium alloys (alloys include
Grade 5, Grade 9, Grade 1, and SP700 (Ti-4.5Al-3V-2Mo-2Fe)); and
(4) copper and copper alloys.
[0048] It should be noted that for reasons of electrochemical
stability, some case materials should only be used at one or the
other of the battery polarities, or at a neutral potential. For
example, for many lithium-based battery configurations, aluminum
may be an inappropriate housing material if the housing 20 is at
the negative polarity of the cell, and stainless steel or titanium
would be inappropriate at the positive cell polarity. This housing
material choice depends on battery chemistry and knowledge of
material stability. FIGS. 20 and 21 illustrate MRI images taken of
a stainless steel battery case (FIG. 20) and an aluminum battery
case (FIG. 21). As shown, the amount of distortion caused by the
aluminum battery case is significantly less than that caused by the
stainless steel battery case.
[0049] Other components of the battery 10 may also be formed of
materials that are less likely to cause significant MRI image
distortion. For example, according to an exemplary embodiment,
components such as the pin 25 (which is conventionally made of a
titanium alloy or niobium), the tabs 34, 38, and the bracket 29 may
be made of aluminum, copper, vanadium, or alloys and combinations
thereof, and the header and fill port 28 may be made one or more of
the materials described above with respect to the housing 20.
[0050] According to an exemplary embodiment in which a battery has
a case negative design, the tab coupled to the negative electrode
(e.g., tab 38 as shown in FIG. 6) may be formed of vanadium or a
vanadium alloy. According to another exemplary embodiment in which
the a battery has a case neutral or case positive design, the pin
25 and/or the tab coupled to the negative electrode (which would be
tab 34 as shown in FIG. 6, since the electrodes would be reversed
in such a configuration) may be formed from vanadium or a vanadium
alloy. According to another exemplary embodiment, the tab coupled
to the positive electrode may be formed from vanadium or a vanadium
alloy where the potential of the positive electrode does not exceed
approximately 3.6 volts (i.e., does not exceed the corrosion
potential of vanadium). Other features of the battery 10 may also
be formed of vanadium or a vanadium alloy according to other
exemplary embodiments, such as the housing 20 or the cover 22.
[0051] One advantageous feature of using vanadium is that in
addition to being beneficial for reducing MRI image distortion,
vanadium may also be welded to both copper and titanium (e.g.,
which would allow a case-negative design with a titanium case such
that vanadium could be welded from a copper electrode to the
titanium case). In other applications, a case-positive design may
be used in which an aluminum interconnect from an aluminum positive
electrode could be welded to an aluminum case.
[0052] The conductors that connect the pin 25 to the hybrid 130 may
be made of aluminum, copper, titanium, vanadium, or alloys thereof.
Alternatively, the pin 25 may be connected directly to the hybrid
130 without the need for separate connectors. In another example,
non-ferrous conductors such as titanium or titanium alloys may be
used to connect the pin 25 to the bal-seals.
[0053] According to an exemplary embodiment in which the pin 25 is
provided for a hermetically sealed battery, the pin may be made of
a titanium alloy (e.g., Grade 5, Grade 9, or SP700). According to
another exemplary embodiment, the pin could also include
aluminum.
[0054] According to an exemplary embodiment in which the pin 25 is
provided for a non-hermetically sealed battery, it may include a
crimped or "rivet feedthrough" with a polymer grommet. The rivet
portion would be the same material as listed above for hermetically
sealed batteries. The polymer may be a polyolefin (e.g., a
low-creep material such as polypropylene or HDPE) or may be
ETFE.
[0055] According to an exemplary embodiment, the negative active
material may be selected from graphite, Li.sub.4Ti.sub.5O.sub.12,
lithium alloying elements such as aluminum, tin, and silicon, and
combinations thereof, and the positive active material may be
selected from LiCoO.sub.2; LiM.sub.xNi.sub.1-xO.sub.2 (where M is a
metal such as Ti or Al); LiMn.sub.2O.sub.4;
LiCo.sub.xMn.sub.yNi.sub.zO.sub.2 (where x+y+z=1), and combinations
thereof.
[0056] Various components of the implantable medical device 100 may
also be made from materials that are less likely to produce
significant MRI image distortion. For example, screws and set screw
blocks conventionally used in assembling the device 100 are formed
of stainless steel, which has a significant ferrous content.
According to an exemplary embodiment, the device 100 utilizes
connectors that are formed from a material (e.g., titanium or a
titanium alloy, a polymeric material such as polysulfone or
polyether ether ketone, etc.) that is both biocompatible and has a
reduced tendency to distort MRI images.
[0057] Implantable medical devices such as that shown in FIG. 4 may
also utilize one or more electrical contacts (not shown) within the
device. Presently, such contacts are provided in the form of
platinum spring contacts (referred to as Bal-seals) inside of
housings made from an alloy including chromium, cobalt, molybdenum,
and nickel. According to an exemplary embodiment, the device 100
utilizes a low permeability stainless steel (e.g., having a
permeability of between approximately 1.008 and 1.02) in place of
the alloy formerly used to produce the housing. According to
another exemplary embodiment, the housing may be made of platinum
or a platinum alloy. According to yet another exemplary embodiment,
the housing may be made of a creep resistant polymer (e.g.,
polysulfone or thermoplastic polyether ether ketone (referred to as
PEEK)). One advantageous feature of utilizing a polymer is that
doing so would not only reduce the image distortion due to the use
of ferrous materials, but it would also eliminate the image
distortion caused by eddy currents. According to still yet another
exemplary embodiment, the housing may be made of a platinum-clad
titanium material.
[0058] The wiring board or hybrid 130 includes various features
such as electronic chips and the like. Conventionally, such
components may utilize ferrite materials (e.g., as cores for coils
or inductors). According to an exemplary embodiment, the hybrid 130
is produced without the use of ferrite materials (e.g., telemetry
and/or recharge antennas may utilize air core antennas).
[0059] Conventional bonding pads utilized with the hybrid 130 may
be formed of nickel 200 or Kovar (an alloy of iron, nickel, and
molybdenum). According to an exemplary embodiment, the bonding pads
utilized with the hybrid may be formed of titanium, platinum,
copper, or alloys thereof.
[0060] The coils 120 utilized with the device 100 have
conventionally been formed of copper, and have included ferrite
cores. Because copper has a permeability close to that of water,
and acts as an excellent conductor, it may be desirable to continue
to use copper for the coils 120. According to an exemplary
embodiment, copper coils with air cores are used in place of the
copper coils with ferrite cores used in conventional designs.
According to other exemplary embodiments, a member or element such
as a ring of shimming material (e.g., platinum, aluminum) may be
provided adjacent the coil or at a location proximate the coil.
[0061] The device enclosure 140 has conventionally been made of
grade 1 titanium, which has a permeability that is relatively close
to that of water. However, because the device enclosure 140 has a
relatively large surface area, relatively significant eddy currents
can be created on it during an MRI scan, which may result in image
distortion. In order to reduce the eddy currents formed during an
MRI scan, the device enclosure may be formed of a titanium having a
higher resistivity. For example, according to an exemplary
embodiment, the device enclosure may utilize a grade 9 titanium (or
another grade or alloy of titanium having a higher
resistivity).
[0062] According to other exemplary embodiments, different
materials may be used to form the device enclosure 140. For
example, according to an exemplary embodiment, the enclosure is
constructed out of a polymer and then covered using a diamond-like
coating for hermeticity. It is expected that such a construction
would significantly reduce the eddy currents created on the
enclosure during a scan and the resulting image distortion.
According to another exemplary embodiment, the device enclosure 140
may be constructed using a creep resistant polymer such as
thermoplastic polyether ether ketone or polysulfone, which would
completely eliminate the eddy currents created on the enclosure
during a scan.
[0063] According to other exemplary embodiments, the device
enclosure 140 may be formed such that is includes a variety of
thicknesses to create reflections that reduce the loop area of the
eddy currents during a scan. For example, the enclosure 140 may be
made with areas (e.g., zones, regions, etc.) that are thinner than
surrounding areas to further increase the electrical resistivity to
minimize eddy currents. According to one exemplary embodiment, the
enclosure 140 may be produced in a metal injection molding process
(e.g., in which a titanium housing is injection molded) such that a
plurality of areas or regions of the housing are formed as having
different thicknesses. According to another exemplary embodiment,
the housing may have a substantially uniform thickness and have
components coupled (e.g., welded) thereto to increase the thickness
of particular regions of the housing (e.g., a titanium housing may
have titanium, platinum, or aluminum strips of material welded
thereto).
[0064] Extensions, tethers, or other accessories may be constructed
using materials that do not cause significant image distortion. For
example, according to an exemplary embodiment, extensions or
tethers may be constructed using platinum irridium filers and
contacts.
[0065] According to an exemplary embodiment, all or a portion of an
implantable medical device such as an INS may be shimmed using
materials having known magnetic permeabilities in order to decrease
image distortion when an organism in which the device 100 is
implanted undergoes an MRI scan. For example, if a device has a
relative magnetic permeability that makes it paramagnetic overall,
it may be advantageous to provide a diamagnetic shimming material
in order to "balance" the paramagnetic character of the device,
thus bringing the overall magnetic permeability of the device
closer to that of water. Shimming may be accomplished in any number
of ways, including coating or plating (e.g., cladding) the device
or portions thereof with a shimming material (e.g., a titanium
housing may be shimmed with platinum or palladium). Another manner
in which shimming may be accomplished is by providing components
within the device that have desired shimming properties. These and
other methods of shimming a medical device will be described
hereafter with reference to the accompanying drawings. Materials
used to shim the device may be selected based on a variety of
considerations including their magnetic permeability
characteristics and biocompatibility, and may include, for example,
titanium, palladium, platinum, silver, copper, manganese, aluminum,
and alloys and combinations thereof.
[0066] While the goal of shimming the implantable medical device
with a shimming material is to produce a device that has a magnetic
permeability as close to water as possible (and thus, as close as
possible to that of the organism into which it will be implanted),
in some circumstances it may be adequate to simply add some
shimming material such that there is less MRI image distortion than
if no shimming material were added. The degree of acceptable MRI
image distortion that is acceptable or suitable in a given
circumstance may vary depending on the location of the device
implantation and other factors, and the amount of shimming material
added to the device should be selected such that it is adequate to
provide a suitably small amount of image distortion under the
circumstances.
[0067] According to an exemplary embodiment, a method of
determining the appropriate amount of shimming material to be added
to an implantable medical device includes creating a prototype of
the device and creating an electromagnetic model of the device
prototype. The prototype may then be analyzed to determine the
aggregate relative magnetic permeability of the device prototype
(e.g., by viewing the field perturbation in a magnetic field).
Based on the aggregate relative magnetic permeability of the device
prototype, it may be determined whether the shimming material
should be paramagnetic or diamagnetic. The appropriate shimming
material may then be selected and added to the device prototype.
After the shimming material is added to the device prototype, the
device may be re-modeled to determine its new aggregate relative
magnetic permeability. Shimming material may again be added and the
device re-modeled in an iterative manner until the desired level of
aggregate magnetic permeability is reached.
[0068] To determine the proper placement of the shimming material,
it is desirable to first understand which components of the device
are most likely responsible for the image distortion (i.e., the
"problem components"). To enhance the ability of the shimming
material to perform its desired function, it may be beneficial to
design the device such that the problem components are not grouped
together, but rather are spread apart. FIG. 7 illustrates a circuit
board or hybrid 200 that includes two problem components 202 and
204. As illustrated, the problem components 202 and 204 are not
provided proximate or adjacent one another, but rather are spread
out such that they are on opposite ends of the hybrid 200 to reduce
the amount of localized image distortion caused by the components.
The particular placement and location of the problem components may
vary according to various exemplary embodiments.
[0069] According to another exemplary embodiment, problem
components may be provided proximate or adjacent to other
components that have opposite magnetic permeabilities (e.g., a
paramagnetic component may be provided next to one or more
diamagnetic components) in order to provide some degree of
localized cancellation of the undesirable effect caused by the
components (e.g., to effectively "shim" the problem components).
For example, as shown in FIG. 7, if component 204 is diamagnetic,
components 206 and 208 may be paramagnetic.
[0070] According to other exemplary embodiments, shimming materials
may be incorporated within other materials used within an
implantable medical device. For example, as shown in FIG. 8, an
implantable medical device 300 includes a housing or casing 302 in
which a hybrid or circuit board 310 is provided. A potting or
filler material 320 may be provided within the device 300, with a
shimming material provided or loaded in the potting material 320.
According to an exemplary embodiment, the potting material is made
of epoxy or silicon rubber and has a suitable shimming material
provided therein (e.g., if the device is paramagnetic overall, a
diamagnetic material such as silver may be incorporated in the
potting material).
[0071] Similar to the embodiment shown in FIG. 8, an epoxy or other
adhesive used to secure various components within the device 300
may include a shimming material provided or loaded therein. One
advantageous feature of providing shimming material in a potting
material or in an adhesive provided within the housing 302 of the
device 300 is that there is little or no impact on the size of the
device, since the shimming materials would be provided in spaces
that are either unoccupied or in materials that are already used in
the device.
[0072] Other components of the device may also be loaded with a
shimming material. For example, as shown in FIG. 4, an insulator
cup 170 is provided in the device 100 for housing certain
components of the device 100. According to an exemplary embodiment,
the insulator cup 170 may be loaded with a shimming material in
order to improve the permeability of the device 100.
[0073] According to other exemplary embodiments, all or a portion
of the device enclosure may be coated, plated, or clad with a
shimming material. FIGS. 9 and 10 illustrate a shimming material
coated (e.g., plated, clad, etc.) on a housing of an implantable
medical device according to two exemplary embodiments. FIG. 9
illustrates a device 400 having a housing or casing 402 with a
shimming material 410 provided on an outer or exterior surface of
the housing 402. FIG. 10 illustrates a device 500 having a housing
or casing 502 with a plating or coating 510 provided on an inner or
interior surface of the housing 502. According to an exemplary
embodiment, the housings shown in FIGS. 9 and 10 are made from a
paramagnetic material, and the shimming material is made from a
diamagnetic material. According to other exemplary embodiments, the
housings may be made of paramagnetic materials and the shimming
material may be a diamagnetic material.
[0074] It should be noted that where the shimming material is
provided on an exterior surface of the device housing, it is
advisable to ensure that the material selected for the plating or
coating material be biocompatible with the organism into which the
device is to be implanted. The thickness of the shimming material
may vary according to various exemplary embodiments. According to
an exemplary embodiment, the shimming material has a thickness of
between approximately 0.005 inches (0.125 mm) and 0.040 inches (1
mm).
[0075] While FIGS. 9 and 10 illustrate a shimming material provided
on either the exterior or interior surface of the housing, it
should be noted that shimming material may be provided on both the
exterior and interior surfaces (and further, that only a portion of
the exterior and/or interior surfaces may have shimming materials
provided thereon). The shimming materials provided on the exterior
surface and the interior surface may have identical or different
compositions. Further, more than one type of shimming material may
be provided on the exterior and/or interior surfaces of the housing
(e.g., silver may be provided on a portion of the exterior surface
and titanium on another portion, etc.).
[0076] According to a particular exemplary embodiment, the interior
and/or exterior of a housing for an implantable medical device is
at least partially coated (e.g., clad, plated, etc.) with titanium
or a titanium alloy to a thickness sufficient to reduce or minimize
image distortion caused by the device enclosure. According to
another particular exemplary embodiment, the interior and/or
exterior of a housing for an implantable medical device is at least
partially coated (e.g., clad, plated, etc.) with silver or a silver
alloy to a thickness sufficient to reduce or minimize image
distortion caused by the device enclosure (e.g., up to
approximately 0.005 inches).
[0077] While FIGS. 9 and 10 illustrate embodiments in which the
entire interior and/or exterior surfaces of the device housings are
coated or plated with a shimming material, it should be noted that
according to other exemplary embodiments, the shimming or plating
material may be provided on only a portion of the housing or
casing. As shown in FIG. 11, an implantable medical device 600
includes a battery 610 and a hybrid or circuit board 620 provided
within a housing or casing 602. The hybrid 620 includes two
components 622 and 624 that are most responsible for producing
image distortion in an MRI image (i.e., they are so-called "problem
components"). Shimming materials are providing on housing 602 only
in locations proximate or adjacent to the problem components 622,
624 (shown as regions or areas of shimming materials 630 and 632).
In this manner, the shimming material is placed in the location
where it may be most efficacious in reducing the amount of MRI
image distortion caused by the problem components. One advantage of
such a configuration is that the amount of shimming material
required to provide adequate shimming of the device may be more
closely tailored to the amount actually needed, which may result in
less shimming material being used than if the entire interior
and/or exterior surface of the housing were coated with the
shimming material. The provision of separate regions of coating or
plating on the device housing may be accomplished using masking
techniques or any other suitable technique.
[0078] The thickness of the shimming material may vary according to
various considerations, including the particular amount of shimming
needed at a particular location within the housing. FIG. 12
illustrates an implantable medical device 700 having a housing 702
with a battery 710 and a hybrid or circuit board 720 provided
therein. In the embodiment shown, the battery 710 has been
determined to be less responsible for MRI image distortion than the
hybrid 720. Accordingly, a greater amount of shimming material is
provided proximate or adjacent the hybrid 720 as compared to that
provided adjacent the battery 710. This is illustrated by a first
plated or coated shimming material 740 having a first thickness
provided adjacent the battery 710 and a second plated or coated
shimming material 750 provided adjacent the hybrid 720. According
to an exemplary embodiment, the two shimming materials 740 and 750
have the same composition. According to other exemplary
embodiments, the shimming materials 740 and 750 have different
compositions (e.g., shimming material 740 may comprise platinum,
while shimming material 750 may comprise aluminum). The composition
of the shimming materials and the coating/plating thicknesses
selected will vary according to various exemplary embodiments in
accordance with the necessary amount of shimming in particular
regions within the housing.
[0079] In certain cases, providing a shimming material in the form
of a coating or plating on the device housing may adversely affect
charging or telemetry of the device. In such cases, it may be
desirable to provide the coatings in a manner that will not affect
such functions. For example, FIGS. 13 and 14 illustrate schematic
views of an implantable medical device 800 according to an
exemplary embodiment. Device 800 includes a housing or casing 802
having a battery 810 and a hybrid or circuit board 820 provided
therein. A recharging or telemetry coil 812 is provided proximate
the battery 810. To reduce or eliminate the adverse effect that may
be caused by coating of the housing 802, coating is not provided in
the areas of the housing proximate the recharging or telemetry coil
812. As shown in FIG. 14, coatings 830 and 832 are provided
adjacent the battery and coatings 834 and 836 are provided adjacent
the hybrid 820. Spaces are provided between coatings 830 and 834
and between coatings 832 and 836 in the regions 840, 842, and no
coating is provided in regions 844 and 846. In this manner, no
coating is provided in regions 840, 842, 844, and 846 proximate the
recharging/telemetry coil. The selective coating of the housing may
be accomplished by masking or other suitable methods.
[0080] The plating or coating of the device housings as shown in
FIGS. 9-14 may be accomplished by any suitable method (e.g., PVD,
CVD, sintering, etc.). According to an exemplary embodiment, a
paramagnetic material is coated on the device housings by physical
vapor deposition. It should also be noted that while FIGS. 11-14
illustrate plating on the inner surface of the device housings,
similar results may be obtained by coating or plating the exterior
surface of such devices (e.g., the coating shown within the
housings in FIGS. 11-14 may instead be provided on an exterior
surface of the housing).
[0081] It should be noted that features shown in FIGS. 8-14 may be
combined as may be desired. For example, the embodiment shown in
FIG. 14 may be modified to include coatings having different
thicknesses (as shown, e.g., in FIG. 12) and/or to have coatings
provided adjacent problem areas (as shown, e.g., in FIG. 11 with
respect to problem areas adjacent the hybrid). A potting material
loaded with a shimming material may also be used. Various
combinations of such features are possible, and all such
combinations are intended to fall within the scope of the present
disclosure.
[0082] Any of a variety of coating methods may be used to provide
shimming material on all or a portion of a component such as the
housing. For example, all or a portion of the housing may be
completely coated with a shimming material. According to other
exemplary embodiments, the coating may be applied as having any of
a variety of patterns using masking or other techniques that are
now known or may be hereafter developed. FIG. 15 illustrates four
coating patterns (formed, e.g., using masking techniques or other
suitable techniques) that may be applied to all or a portion of a
component according to various exemplary embodiments, although it
should be understood that numerous other patterns may be possible.
Additionally more than one different type of pattern may be applied
to the same component.
[0083] As an alternative to (or in addition to) plating or coating
the housing with a shimming material, pieces (e.g., members,
elements, etc.) of shimming material may be coupled to the housing
by welding, soldering, adhesive, or by any other suitable method.
FIG. 16 illustrates a schematic cross-sectional view of an
implantable medical device 900 having a housing 902 having a hybrid
or circuit board 910 provided therein. Two pieces of shimming
material 920 and 922 are coupled to an interior surface of the
housing 902 in areas unoccupied by components of the device 900.
Any number of shimming components may be coupled to the housing
having any of a variety of sizes, shapes, and/or configurations
according to various exemplary embodiments.
[0084] FIG. 17 illustrates a schematic plan view of an implantable
medical device 1000 according to another exemplary embodiment. The
device 1000 includes a hermetically sealed housing or casing 1002
that has rails 1010 and 1020 welded to the exterior surface
thereof. The rails 1010 and 1020 are made of titanium according to
an exemplary embodiment and have a shimming material loaded into
the rails. According to an exemplary embodiment, the rails have a
thickness of between approximately 1 mm and 4 mm, although the
size, shape, and/or configuration of the rails may vary according
to other exemplary embodiments. According to other exemplary
embodiments, the rails may be coupled to an interior surface of the
housing. According to an exemplary embodiment, the shimming
material is encapsulated in a biocompatible material to avoid
direct tissue contact and corrosion.
[0085] Other shimming components can also be provided exterior to
the housing or casing of an implantable medical device. For
example, FIG. 18 illustrates an implantable medical device 1100
according to another exemplary embodiment having a shimming
component 1110 coupled to an exterior surface of its housing or
casing 1102. According to an exemplary embodiment, the shimming
component 1110 is a polymeric material loaded with a shimming
material that is molded directly to the housing 1102 (e.g., where a
housing of a device is made of titanium or a titanium alloy, the
polymeric material may be an epoxy loaded with a diamagnetic
material such as silver or a silver alloy, although the particular
constituents may differ according to other exemplary embodiments;
additionally, the polymeric material may be provided on the
interior surface of the housing). According to another exemplary
embodiment, the component 1110 is a boot that surrounds the housing
1102 (e.g., the boot may be a silicon rubber (or similar material)
boot that has a shimming material incorporated therein). According
to yet another exemplary embodiment, the component 1110 is a
paralyne coating that has incorporated therein a shimming material
(again, it should be noted that the paralyne coating may be
provided on the interior surface of the housing). It should be
noted that while FIG. 18 illustrates component 1110 as completely
surrounding the housing 1102, according to other exemplary
embodiments, the component 1110 may only partially surround the
housing 1102 (e.g., the component may be provided at only select
locations on the exterior surface of the housing 1102).
[0086] As an alternative to or in addition to shimming all or a
portion of the medical device housing or casing, it may also be
possible to provide shimming materials on structures or elements
included within the medical device housing. For example, because
the battery 10 is provided within the device enclosure 140, and is
therefore not in direct contact with the human body in which the
device 100 is implanted, it may be desirable to plate or coat the
exterior surface of the battery 10 with a shimming material (e.g.,
a diamagnetic material). According to an exemplary embodiment, at
least a portion of an interior or exterior surface of a titanium or
titanium alloy battery housing 20 is coated (e.g., clad, plated,
etc.) with silver or a silver alloy to a thickness sufficient to
reduce image distortion caused by the housing 20 (e.g., between
approximately 0.0001 and 0.01 inches). According to another
exemplary embodiment in which a battery housing 20 is made of
copper or a copper alloy, the housing 20 may be coated (e.g., clad,
plated, etc.) with titanium or titanium alloy to a thickness
sufficient to reduce image distortion caused by the housing 20
(e.g., between approximately 0.0001 and 0.01 inches). It should be
noted that all or a portion of the interior and/or exterior surface
of the battery housing may be plated or coated with a shimming
material, and that the coating and plating methods described herein
with respect to FIGS. 8-18 may be utilized as may be appropriate in
a particular battery configuration (e.g., two or more portions of
the housing may be plated with different amounts of shimming
material, as shown for example in FIG. 12 in the context of an
implantable medical device).
[0087] According to another exemplary embodiment, a non-functional
shimming component (e.g., a shimming member or element) may be
added to a hybrid or circuit board (e.g., hybrid 200) to shim the
hybrid. For example, as shown in FIG. 7, a shimming component 210
(e.g., member, element, etc.) is added directly to the hybrid in
order to shim the hybrid. The component 210 serves no electronic
purpose for the hybrid, and in effect is a "dummy" component (e.g.,
a block of shimming material) provided for the purpose of shimming
the hybrid 200. The component 210 may be provided in any suitable
location on the hybrid (e.g., adjacent to another component that
would benefit from being shimmed, in a blank area on the hybrid,
etc.). The size, shape, and/or configuration of the component 210
may vary according to other exemplary embodiments. It should also
be noted that more than one such component may be provided on the
hybrid. One advantageous feature of providing shimming components
directly on the hybrid is that the size of the device need not be
increased to accommodate the shimming component (since the hybrid
has a fixed size, and the shimming components fit in otherwise
unused space on one or both sides of the hybrid).
[0088] According to another exemplary embodiment, an implantable
medical device includes a header block that includes a number of
metallic components for use by the device. FIG. 19 is a schematic
plan view of a device 1200 that includes a header block 1210.
Shimming components 1220 and 1230 may be provided within the header
block to shim other metallic components provided in the header
block and/or to shim the device as a whole. One advantageous
feature of providing shimming components within the header block is
that the shimming components are provided in relatively close
proximity to the metallic components in the header block.
[0089] The type of MRI scan can also be designed to reduce
distortion. Different types of scans can have different effects on
the image distortion. For example, a gradient echo pulse sequence
is disadvantageous for mitigating image distortion and a spin echo
sequence is superior. Further, a technique called shimming can also
be used to address image distortion. Image distortion is better on
lower static magnetic field strength machines and is worse on
higher static magnetic field strength machines. Ways to minimize
the image distortion through the scan type, image processing, and
shimming will be described in detail below.
[0090] According to an exemplary embodiment, the MRI machine and
associated image processing system are used to address the image
distortion resulting from an implant, such as a cranial implant.
The adjustments may further reduce the distortion resulting from
such a device after applying one or more of the other features of
the invention discussed above. In particular, the pulse sequence
selection, certain image processing techniques, and shimming may
all be used to address or counteract distortions resulting from the
implant.
[0091] Image artifacts from metallic implants are highly dependent
on the choice of pulse sequence. The particular MRI pulse sequence
may be selected to reduce image distortion. There are several
commonly used pulse sequences in diagnostic MRI imaging. These
include gradient echo, spin echo, and fast spin echo pulse
sequences, among others. Spin-echo pulse sequences are inherently
less susceptible to magnetic field distortion artifacts relative to
gradient echo sequences. Therefore, a spin echo sequence may be
selected to reduce image distortion associated with the
implant.
[0092] Image processing techniques may also be utilized to address
image distortion associated with an implanted cranial device. For
example, image processing software may be used to correct for known
distortions. In addition to a general application of image
processing techniques to correct for distortion, in a preferred
embodiment, techniques are utilized that are applicable to a
specific implanted device. Such techniques may include using known
factors, such as material composition, size, and implanted
location, in a processing algorithm to compensate for distortion
associated with a specific implanted device. In practice, the
necessary image processing algorithm may be determined
experimentally by acquiring images of a specific device implanted
in a phantom to provide the necessary correction data.
[0093] Yet another imaging approach that may be utilized to address
cranial image distortion is the use of a wider bandwidth for the
imaging gradients.
[0094] Shimming techniques may also be utilized to address magnetic
field inhomogeneity due to an implant, such as device 100. Ideally,
the magnetic field gradients associated with the presence of an
implanted device may be addressed through the use of shim coils to
correct the field inhomogeneity. Shim coils are designed and placed
on an MRI or an NMR machine to actively compensate for magnetic
field inhomogeneities, doing so by providing "canceling" magnetic
fields when current is run through the shim coils. Adjusting the
currents in such shim coils to cancel the unwanted gradients is
known as "active shimming." Various active shimming approaches are
known and used, especially in NMR spectroscopy. However, there is a
need for the ability to correct for magnetic field inhomogeneities
in MRI, and certain NMR shimming principles and devices may be
applied in accordance with the present invention.
[0095] Certain conventional MRI machines utilize shim coils to
correct for the magnetic field inhomogeneity due to placement of
the patient in the field. The shim coils are intended to address
first order gradients due to the patient, i.e., to correct for the
linear variation in magnetic field strength with position due to
the presence of the patient in the field. Such shim coils may be
useful to adjust the field inhomogeneities due to an implant using
approaches in accordance with certain embodiments of the present
invention. However, implants, especially those including
ferromagnetic materials, may result in higher order gradients,
e.g., second order gradients having quadratic variations in field
strength. The active shim coils presently in use on MRI machines to
correct first order gradients may not suitably address the second
order gradients associated with an implant. Accordingly, in an
embodiment of the present invention, additional shim coils are
installed and utilized to correct for the presence of the implant.
Likewise, additional shim coils may be utilized to correct higher
order gradients, depending on the desired level of correction.
[0096] Active shimming may be performed manually by changing the
current supplied to various shim coils to achieve the best gradient
correction. However, computers have been used to automate such
shimming with varying degrees of success because the process of
shimming tends to be complex and tedious, especially due to the
interactions between multiple shim coils when correcting second and
higher order gradients.
[0097] According to an exemplary embodiment, a custom shim for the
implant is utilized. The custom shim may be created because the
device properties are known and therefore may be specifically
addressed by an active shimming technique. In a preferred
embodiment, custom software may be utilized to automate the active
shimming, and may be customized for a specific cranial device,
and/or a specific MRI machine.
[0098] According to an exemplary embodiment, the distortion
associated with the implant is mitigated as follows. A patient
having an implant is positioned in an MRI machine for image
acquisition. Prior to imaging, shim coils are used to adjust the
magnetic field to address unwanted gradients in the field caused by
the implant (and patient). The adjustment may include the use of
shim coils to address first order gradients or may involve
adjusting the current in multiple shim coils to address first
order, second order, and even higher order gradients. Once the
field inhomogeneities have been addressed via active shimming
(either manually, or automatically, utilizing custom shimming
software or other automating techniques), the image may be
acquired, preferably by utilizing a spin echo pulse sequence. Once
the data has been acquired, the image may be reconstructed using
conventional reconstruction techniques that are modified to adjust
for known distortions associated with the implant, such as the use
of a custom image processing routine to adjust for the specific
implanted device.
[0099] While the implantable medical device is described herein in
the context of an implantable neurological stimulation device, it
should be understood that other types of implantable medical
devices may also benefit from the use of the features described
herein, including, by way of example, implantable defibrillators,
pacemakers, cardioverters, cardiac contractility modulators, drug
administering devices, diagnostic recorders, cochlear implants, and
the like.
[0100] It is important to note that the construction and
arrangement of the implantable medical devices as shown in the
various exemplary embodiments is illustrative only. Although only a
few embodiments have been described in detail in this disclosure,
those skilled in the art who review this disclosure will readily
appreciate that many modifications are possible (e.g., variations
in sizes, dimensions, structures, shapes and proportions of the
various elements, values of parameters, mounting arrangements, use
of materials, colors, orientations, etc.) without materially
departing from the novel teachings and advantages of the subject
matter described herein. For example, elements shown as integrally
formed may be constructed of multiple parts or elements, the
position of elements may be reversed or otherwise varied, and the
nature or number of discrete elements or positions may be altered
or varied. Accordingly, all such modifications are intended to be
included within the scope of the present invention. The order or
sequence of any process or method steps may be varied or
re-sequenced according to alternative embodiments. Other
substitutions, modifications, changes and omissions may be made in
the design, operating conditions and arrangement of the various
exemplary embodiments without departing from the scope of the
present inventions.
* * * * *
References